1. Technical Field of the Invention
This invention relates to a display device, specifically to a display device which is incorporated into a portable communication and computing device.
2. Background of the Invention
There has been a great demand in the market for portable communication and computing devices such as a portable TV and a cellular phone. All these devices need a small, light-weight and low-power consumption display device, and efforts have been made accordingly.
FIG. 6 shows a circuit diagram corresponding to a single pixel element of a conventional liquid crystal display device. A gate signal line 51 and a drain signal line 61 are placed on an insulating substrate (not shown) perpendicular to each other. A thin-film transistor (TFT) 65 connected to two signal lines 51, 61, is formed near the intersection of the two signal lines 51, 61. A source 11s of the TFT 65 is connected to a display electrode 80 of a liquid crystal 21.
A storage capacitor element 85 holds the voltage of the display electrode 80 during one field period. One terminal 86 of the storage capacitor 85 is connected to the source 11s of the TFT 65 and the other terminal 87 is provided with a voltage common among all the pixel elements. When a scanning signal is applied to the gate signal line 51, the TFT 65 turns to an on-state. Accordingly, an analog image signal from the drain signal line 61 is applied to the display electrode 80, and the storage capacitor 85 holds the voltage. The voltage of the image signal is applied to the liquid crystal 21 through the display electrode 80, and the liquid crystal 21 aligns in response to the applied voltage for providing a liquid crystal display image. This configuration is capable of showing both moving images and still images. There is a need for the display to show both a moving image and a still image within a single display. One such example is to show a still image of a battery within an area of a moving image of a cellular phone display to show the remaining amount of the battery power.
However, the configuration shown in FIG. 6 requires a continuous rewriting of each pixel element with the same image signal at each scanning in order to provide a still image. This is basically to show a still-like image in a moving image mode, and the scanning signal needs to activate the TFT 65 at each scanning. Accordingly, it is necessary to operate a driver circuit which generates a driver signal for the scanning signals and the image signals, and an external LSI which generates various signals for controlling the timing of the driver circuit, resulting in a significant electric power consumption. This is a considerable drawback when such a configuration is used in a cellular phone device which has only a limited power source. That is, the time a user can use the telephone under one battery charge is considerably decreased.
Japanese Laid-Open Patent Publication No. Hei 8-194205 discloses another configuration for a display device suitable for portable applications. This display device has a static memory for each of the pixel elements, as shown in FIG. 7. A static memory, in which two inverters INV1 and INV2 are positively fed back to each other, holds the image signal. This results in reduced power consumption. In this configuration, a switching element 24 controls the resistance between a reference line and a display electrode 80 in response to the divalent digital image signal held by the static memory in order to adjust the biasing of the liquid crystal 21. The common electrode, on the other hand, receives an AC signal Vcom. Ideally, this configuration does not need to refresh the memory when the image stays still for a period of time.
As described above, the display device equipped with a retaining circuit (a static memory) for holding digital image signals is suitable for displaying a still image with a shallow depth and reducing the consumption of the electric power.
However, the aforementioned liquid crystal display device has the following problem. The problem will be explained hereinafter by referring to FIG. 8. Suppose a “L (low)” level is held at a source 11s of a pixel element selection TFT 72, and a “H (high)” level is held at the output node of an inverter INV1.
Under this condition, when the external circuit outputs “H” to the drain signal line 61 to write “H” into the static memory, the N channel TFT of an inverter INV2 is on. Thus, as shown by the broken line in FIG. 8a, the electric current from the drain signal line 61 goes through the pixel element selection TFT 72 and reaches the N channel TFT. Therefore, the “H” level competes against “L” level, resulting in a reduced “H” level, and possibly causing a false writing. In order to achieve the correct writing of the “H” data, the voltage of the source 11s of the pixel element selection TFT 72 should be higher than the threshold voltage of the inverter INV1. However, because of the presence of the electric passage described above, this condition may not be fulfilled due to the decrease in the voltage of the source 11s of the pixel element selection TFT 72.
The similar problem takes place when the “L” level is held at the output node of the INV1 (see FIG. 8b). Under this condition, when the external circuit outputs the “L” to the drain signal line 61 to write the “L” into the static memory, the P channel TFT of the inverter INV2 is on. Thus, as shown by the broken line in FIG. 8b, the electric current goes through the P channel TFT and then to the pixel element selection TFT 72 to reach the drain signal line 61. In order to achieve the correct writing of “L” data, the voltage of the source 11s of the pixel element selection TFT 72 should be lower than the threshold voltage of the inverter INV1. However, because of the presence of the electric passage described above, the condition mentioned above may not be fulfilled due to the increase of the voltage of the source 11s of the pixel element selection TFT 72.
Accordingly, a conventional device such as shown in FIG. 8 may not properly change the status of the retaining circuit, or may take a long time to change the status.